This invention relates to the technical field of flexible printed wiring boards. More particularly, it relates to a technique for producing flexible printed wiring boards enabling the formation of fine conductive bumps.
There have been frequently employed flexible printed wiring boards carrying desired circuit patterns printed thereon. In recent years, flexible printed wiring boards in various shapes are required corresponding to the shapes of the parts where these flexible printed wiring boards are to be used.
FIG. 11a shows an arrangement plan for cutting out T flexible printed wiring boards 152 from a rectangular master sheet 150. In this case, six flexible printed wiring boards 152 can be obtained.
In cutting out specially shaped flexible printed wiring boards as 152 in the above case, however, it is frequently observed that the master sheet 150 is much wasted.
In the conventional art, therefore, attempts have been made to take a flexible printed wiring board in a complicated shape apart into elemental pieces in simple shapes and then bond these pieces together to give a flexible printed wiring board. In FIG. 11c, a flexible printed wiring board 155 having the same shape as the flexible printed wiring board 152 is formed by bonding two rectangular elemental pieces 153 and 154 to each other.
As FIG. 11b shows, the master sheet 150 can be efficiently utilized by cutting out the elemental pieces 153 and 154 in simple shapes therefrom. In the case of FIG. 11b, eight elemental pieces 153 and 154 can be respectively obtained. By bonding these pieces to each other, therefore, eight flexible printed wiring boards 155 can be formed. Namely, the flexible printed wiring boards can be thus obtained in a larger number than the case when the T flexible printed wiring boards 152 are directly cut out.
To bond plural elemental pieces to each other to thereby form a flexible printed wiring board, it is necessary to mechanically and electrically connect these elemental pieces to each other.
The elemental pieces 153 and 154 as described above are connected to each other via conductive bumps preliminarily formed on the master sheet 150. Now, a method for producing the master sheet 150 by the conventional art will be described.
In FIG. 10a, 113 stands for a supporting film made of polyimide and a metal wiring circuit 112 made of a patterned copper foil is adhered onto the surface of the supporting film 113. Further, a cover lay 111 made of a polyimide film is adhered onto the copper foil 112.
First, the cover lay film 111 is irradiated at the definite position with laser beams 114 (FIG. 10b) to form plural openings 115 (FIG. 10c) (FIG. 10c shows only one opening 115.). The supporting film 113 is provided with connecting openings 123 in which the bottom face of the metal wiring circuit 112 is exposed (only one connecting opening 123 is shown in each FIG. 10axcx9cd). In each opening 115 formed above, the surface of the metal wiring circuit 112 is exposed.
Subsequently, a protective film is formed on the back face of the supporting film 113 to protect the connecting openings 123. After copper-plating, the protective film is stripped off. Thus, copper grows within each opening 115 by the copper-plating and thus conductive bumps 116 are formed (FIG. 10d).
From the master sheet 150 in the above-described state, elemental pieces 153 and 154 are cut out. In FIG. 10e, 153 and 154 stand for the elemental pieces thus cut out wherein members of these two elemental pieces 153 and 154 are each distinguished from the corresponding one by a or b.
The conductive bump 116b of the elemental piece 154 (i.e., one of the two elemental pieces 153 and 154) is located toward the connecting opening 123a of the other elemental piece 153. The tip of the conductive bump 116b is brought into contact with the metal wiring circuit 112a exposed in the connecting opening 123a via an anisotropic conductive film 160. Thus, these two elemental pieces 153 and 154 are adhered to each other due to the anisotropic conductive film 160 thereby giving a specially shaped flexible printed wiring board 155.
The metal wiring circuits 112a and 112b serving as two layers of this flexible printed wiring board 155 are electrically connected to each other via conductive particles dispersed in the anisotropic conductive film 160 , while the two elemental pieces 153 and 154 are adhered to each other owing to the adhesiveness of the anisotropic conductive film 160.
When a semiconductor chip such as an integrated circuit device is to be packaged in the above-described flexible printed wiring board 155 , the anisotropic conductive film is located on the conductive bump 116a and then a bonding pad of the semiconductor device is brought into contact with the conductive bump 116 via the anisotropic conductive film followed by bonding. The inner circuit of the semiconductor device is connected to the metal wiring circuits 112a and 112b via the conductive particles in the anisotropic conductive film and the conductive bumps 116a and 116b. 
By adhering such elemental pieces as the above-described ones 153 and 154, it is possible to obtain flexible printed wiring boards in a desired shape which are thin, light and freely bendable as the one 155. Therefore, this technique has been frequently employed in recent years.
When the openings 115 are formed by using laser beams 114 as in the above case, however, the residue of the polyimide film 111 remains in the surface of the metal wiring circuit 112 exposed on the bottom of the openings 115. In the conventional art, therefore, the elemental piece is soaked in a chemical solution, after the formation of the openings 115, so as to eliminate the residue therefrom. As the openings 115 become finer, however, the chemical solution can hardly enter the openings 115 and thus the residue can be hardly eliminated.
When the residue cannot be eliminated, the copper deposition speed varies from opening to opening and, in its turn, uniform conductive bumps 116 cannot be formed.
Since the opening 115 is formed by irradiating a rigid polyimide film (i.e., the cover lay 111) with laser beams 114, the opening size varies, when the opening is fine (diameter about 40 to 50 xcexcm). As a result, the diameter and height of the thus formed conductive bump varies, which causes a contact failure with the semiconductor. Although attempts have been made recently to form finer opening 115, it is difficult to stop down the high output laser beams 114. It is therefore impossible to form the opening 115 having diameter less than 40 xcexcm.
Moreover, there arises another problem that the adhesion of the elemental pieces 153 and 154 to each other with the anisotropic conductive film 160 makes the flexible printed wiring board 155 expensive.
The present invention, which has been made to overcome the above-described troubles encountering in the prior art, aims at providing a flexible printed wiring board at a low cost by using finely patterned elemental pieces.
To achieve those objects, the present invention relates to an elemental piece of a flexible printed wiring board having a metal wiring circuit patterned into a definite shape, a supporting film located in the side of one face of said metal wiring circuit, and a resin film located in the side of the other face of said metal wiring circuit, wherein said supporting film is provided with at least one connecting opening in which the surface of said metal wiring circuit is exposed and at least one conductive bump connected to said metal wiring circuit projects on said resin coating.
The present invention relates to said elemental piece, wherein said resin film surface has an adhesiveness.
The present invention relates to said elemental piece, wherein said supporting film is made of polyimide.
The present invention relates to said elemental piece, wherein a surface-roughed layer is formed on the surface of said supporting film.
The present invention relates to a flexible printed wiring board having at least two said elemental pieces, wherein the tip of said conductive bump of one elemental piece is in contact with the metal wiring circuit surface exposed in said connecting opening of the other elemental piece, and said elemental pieces are adhered to each other due to the adhesiveness of said resin film of the former elemental piece.
The present invention relates to a flexible printed wiring board having said flexible printed wiring board, and a semiconductor device provided with at least one conductive bump connected to an internal circuit, wherein the conductive bump of said semiconductor device is connected to said metal wiring circuit surface exposed in said connecting opening of said flexible printed wiring board via an anisotropic conductive film.